The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads (also called writers and sensors), a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization of the MR element, which in turn causes a change in resistance of the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the GMR sensor varies as a function of the spin-dependent transmission of the conduction electrons between ferromagnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the ferromagnetic and non-magnetic layers and within the ferromagnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Ni—Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer (reference layer), has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe—Mn) layer. The pinning field generated by the antiferromagnetic layer should be greater than demagnetizing fields (about 200 Oe) at the operating temperature of the SV sensor (about 120° C.) to ensure that the magnetization direction of the pinned layer remains fixed during the application of external fields (e.g., fields from bits recorded on the disk). The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a SV sensor operating on the basis of the GMR effect.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. FIG. 1A shows a prior art SV sensor 100 comprising a free layer (free ferromagnetic layer) 110 separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer layer 115. The magnetization of the pinned layer 120 is fixed by an antiferromagnetic (AFM) layer 130.
FIG. 1B shows another prior art SV sensor 150 with a flux keepered configuration. The SV sensor 150 is substantially identical to the SV sensor 100 shown in FIG. 1A except for the addition of a keeper layer 152 formed of ferromagnetic material separated from the free layer 110 by a non-magnetic spacer layer 154. The keeper layer 152 provides a flux closure path for the magnetic field from the pinned layer 120 resulting in reduced magnetostatic interaction of the pinned layer 120 with the free layer 110. U.S. Pat. No. 5,508,867 granted to Cain et al. discloses a SV sensor having a flux keepered configuration.
Another type of SV sensor is an antiparallel (AP)-pinned SV sensor. In AP-Pinned SV sensors, the pinned layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation. The AP-Pinned SV sensor provides improved exchange coupling of the antiferromagnetic (AFM) layer to the laminated pinned layer structure than is achieved with the pinned layer structure of the SV sensor of FIG. 1A. This improved exchange coupling increases the stability of the AP-Pinned SV sensor at high temperatures which allows the use of corrosion resistant antiferromagnetic materials such as NiO for the AFM layer.
Referring to FIG. 1C, an AP-Pinned SV sensor 180 typically comprises a free layer 182 separated from a laminated AP-pinned layer structure 185 by a nonmagnetic, electrically-conducting spacer layer 184. The magnetization of the laminated AP-pinned layer structure 220 is fixed by an AFM layer 196. The laminated AP-pinned layer structure 220 comprises a first ferromagnetic layer 192 and a second ferromagnetic layer 186 separated by an antiparallel coupling layer (APC) 190 of nonmagnetic material. The two ferromagnetic layers 192, 186 (FM1 and FM2) in the laminated AP-pinned layer structure 185 have their magnetization directions oriented antiparallel, as indicated by the arrows 194, 188 (arrows pointing out of and into the plane of the paper respectively).
As mentioned above, AP-Pinned SV sensors typically use an AFM layer in order to pin the magnetization so that the pinned layers do not move around when the head is reading data from the disk, upon application of external magnetic fields, etc. The AFM layers are typically very thick, on the order of 150-200 Å. Due to the large overall thickness, such sensors are typically not practical for use in applications where a thin head is desirable.
Another type of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetizations of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetization fixed, or pinned, and the other ferromagnetic layer has its magnetization free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect.
FIG. 2A shows a prior art MTJ sensor 200 comprising a first electrode 204, a second electrode 202, and a tunnel barrier layer 206. The first electrode 204 comprises a pinned layer (pinned ferromagnetic layer) 212, an antiferromagnetic (AFM) layer 214, and a seed layer 216. The magnetization of the pinned layer 212 is fixed through exchange coupling with the AFM layer 214. The second electrode 202 comprises a free layer (free ferromagnetic layer) 208 and a cap layer 210. The free layer 208 is separated from the pinned layer 212 by a nonmagnetic, electrically insulating tunnel barrier layer 206. In the absence of an external magnetic field, the free layer 208 has its magnetization oriented in the direction shown by arrow 220, that is, generally perpendicular to the magnetization direction of the pinned layer 212 shown by arrow 218 (tail of an arrow pointing into the plane of the paper). A first lead 222 and a second lead 224 formed in contact with first electrode 204 and second electrode 202, respectively, provide electrical connections for the flow of sensing current Is from a current source 226 to the MTJ sensor 200. Because the sensing current is perpendicular to the plane of the sensor layers, the MTJ sensor 200 is known as a current-perpendicular-to-plane (CPP) sensor. A signal detector 228, typically including a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to the first and second leads 222 and 224 senses the change in resistance due to changes induced in the free layer 208 by the external magnetic field.
FIG. 2B shows an air bearing surface (ABS) view, not to scale, of a dual magnetic tunnel junction (MTJ) sensor 230. The MTJ sensor 230 comprises end regions 234 and 236 separated from each other by a central region 232. The seed layer 244 is a layer deposited to modify the crystallographic texture or grain size of the subsequent layers, and may not be needed depending on the subsequent layer. A first MTJ stack deposited over the seed layer 244 comprises a first antiferromagnetic (AFM1) layer 246, a first AP-pinned layer 247, an electrically insulating tunnel barrier layer 254 and a first sense layer 255. The first AP-pinned layer 247 is formed of two ferromagnetic layers 248 and 252 separated by an antiparallel coupling (APC) layer 250. The APC layer is formed of a nonmagnetic material, preferably ruthenium (Ru) that allows the two ferromagnetic layers 248 and 252 to be strongly antiparallel-coupled together. The AFM1 layer 246 has a thickness at which the desired exchange properties are achieved, typically 100-300 Å.
A longitudinal bias stack sequentially deposited over the first MTJ stack comprises a first decoupling layer 259, a first ferromagnetic (FM1) layer 260, a third antiferromagnetic (AFM3) layer 262, a second ferromagnetic (FM2) layer 264 and a second decoupling layer 263. A second MTJ stack deposited over the longitudinal bias stack comprises a second sense layer 269, a second tunnel barrier layer 270, a second AP-pinned layer 271 and an antiferromagnetic (AFM2) layer 278. The second AP-pinned layer 271 is formed of two ferromagnetic layers 272 and 276 separated by an antiparallel coupling (APC) layer 274. The APC layer is formed of a nonmagnetic material, preferably ruthenium (Ru) that allows the two ferromagnetic layers 272 and 276 to be strongly antiparallel-coupled together. The AFM2 layer 278 has a thickness at which the desired exchange properties are achieved, typically 100-300 Å. A cap layer 280, formed on the AFM2 layer 278, completes the central region 236 of the dual SV sensor 230.
The AFM1 layer 246 is exchange-coupled to the first AP-pinned layer 247 to provide a pinning magnetic field to pin the magnetizations of the two ferromagnetic layers of the first AP-pinned layer perpendicular to the ABS as indicated by an arrow tail 249 and an arrow head 253 pointing into and out of the plane of the paper, respectively. The first sense layer 255 has a magnetization 257 that is free to rotate in the presence of an external (signal) magnetic field. The magnetization 257 of the first sense layer 255 is preferably oriented parallel to the ABS in the absence of an external magnetic field.
The AFM2 layer 278 is exchange-coupled to the second AP-pinned layer 271 to provide a pinning magnetic field to pin the magnetizations of the two ferromagnetic layers of the second AP-pinned layer perpendicular to the ABS as indicated by an arrow head 273 and an arrow tail 275 pointing out of and into the plane of the paper, respectively. The second sense layer 269 has a magnetization 267 that is free to rotate in the presence of an external (signal) magnetic field. The magnetization 267 of the second sense layer 269 is preferably oriented parallel to the ABS in the absence of an external magnetic field.
The AFM3 layer 262 is exchange-coupled to the FM1 layer 260 and the FM2 layer 264 to provide pinning fields to pin the magnetizations 261 and 265, respectively, parallel to the plane of the ABS. The magnetizations 261 and 265 provide longitudinal bias fields which form flux closures with the first and second sense layers 255 and 269, respectively, to stabilize the first and second sense layers 255 and 269.
A major drawback to the MTJ sensors described above is that the AFM layers result in a very thick structure that is not practical for use in modern high density magnetic storage systems.
There is a continuing need to increase the MR coefficient and reduce the thickness of sensors while improving sensor stability. An increase in signal variations in the sensing current and reduced sensor geometry equates to higher bit density (bits/square inch of the rotating magnetic disk) read by the read head.